Effect of Annealing Rates on Surface Roughness of TiO2 Thin films

Abstract

In this work, we report a simple and novel method of determining the morphological properties of TiO2 thin films synthesized by sol-gel doctor-blade method. The TiO2 films were deposited on doped fluorine tin oxide (SnO2:F) layer on glass substrates. The as-deposited and subsequent annealed films at different rates (1-step annealing, 1°C/min and 2°C/min) were studied using optical microscopy. Analysis of the optical images through line scans and histogram distributions, revealed the average surface roughness to be 0.1246±0.0114, 0.1442±0.0069, 0.1393±0.0084, and 0.1333±0.0084μm for the as-deposited, 1-step annealed, 2°C/min, and 1°C/min films, respectively. The average sizes of islands forming on the films were found to be 0.1037± 0.0054, 0.1262±0.0053, 0.1684±0.0103, and 0.1947±0.0078μm for the as-deposited, 1-step annealed, 2°C/min, and 1°C/min films, respectively. A striking statistical correlation between surface roughness and annealing rate was established, with the 1°C/min annealed film having the lowest roughness and the largest size of islands. This is attributed to the dynamics of crystallization where TiO2have more time to partially melt and coalesce into relatively smooth surface at low annealing rate, in contrast to high rates. Our results clearly demonstrate that the surface roughness on TiO2 films heat-treated at different temperatures can be accurately and rapidly determined based on their optical contrast.